This invention relates to FET oscillators in which the oscillation frequency is relatively independent of supply voltage and ambient temperature. The term FET is used to refer to CMOS, MOSFET, JFET and other variation of the Field Effect Transistor.
One of the problems associated with FET oscillators at high frequency is that the oscillation frequency is very sensitive to changes in ambient temperature and power supply voltage. To reduce this instability some form of compensation is necessary. One of the methods used is to use a FET as a resistor to control the charging time of a capacitor. The FET resistance value is controlled by a temperature dependent voltage which varies to maintain a constant capacitor charging time. This is described in U.S. Pat. No. 4,547,749 issued to Clinton Kuo. Another method is to use a constant current source circuit, which is designed to be temperature independent, to charge and discharge a timing capacitor. This is described in U.S. Pat No. 4,714,901 issued to Jain et al.
In these methods the variation in oscillator frequency has been reduced by controlling the charging time of capacitors, but nothing has been done to correct an other large error source, the high sensitivity of the FET amplifier to temperature and supply voltage change.
A solution to this was taught in U.S. Pat. No. 5,241,286 issued to Mirow on Aug. 31, 1993. Mirow taught a oscillator in which the frequency stability is increased by reducing the change in the amplifier circuit gain and phase shift due to variations in ambient temperature and power supply voltage. This reduction is accomplished by powering the amplifier from a power supply in which the output voltage level varies with temperature. However Mirow did not take into account non linear frequency shift or show how to reduce errors in the supply voltage verses temperature curve.